Eukaryotic initiation factor 4E (eIF4E) is a general translation factor, but it has the potential to enhance preferentially the translation of messenger RNAs (mRNAs) that lead to production of malignancy-associated proteins. This selectivity may relate to an increased requirement for eIF4E and its binding partners for the translation of mRNAs containing extensive secondary structure in their 5′-untranslated regions (5′-UTRs). These mRNAs include those encoding certain proteins that control cell cycle progression and tumorigenesis. Under normal cellular conditions the translation of these malignancy-associated mRNAs is suppressed as the availability of active eIF4E is limited; however, their levels can increase when eIF4E is over-expressed or hyperactivated. Elevated levels of eIF4E have been found in many types of tumors and cancer cell lines including cancers of the colon, breast, bladder, lung, prostate, gastrointestinal tract, head and neck, Hodgkin's lymphomas and neuroblastomas.
Initiation of cap-dependent translation is thought to depend on the assembly of eIF4F, an initiation factor complex including eIF4E, the scaffold protein eIF4G, and the RNA helicase eIF4A. Because eIF4E is the only one of these proteins that binds directly to the mRNA cap structure, it is the key factor for the assembly of eIF4F at the 5′ cap. The scaffold protein, eIF4G, also recruits the 40S ribosomal subunit to the mRNA via its interaction with eIF3 and binds eIF4B, a protein that aids the RNA-helicase function of eIF4A, thus facilitating the translation of mRNAs that contain structured 5′-UTRs. The availability of eIF4E as part of the eIF4F complex is a limiting factor in controlling the rate of translation, and therefore eIF4E is an important regulator of mRNA translation.
Regulation of eIF4E activity forms a node of convergence of the PI3K/Akt/mTOR and Ras/Raf/MAPK signaling pathways. The PI3K (phosphoinositide 3-kinase)/PTEN (phosphatase and tensin homologue deleted on chromosome ten)/Akt/mTOR (mammalian target of rapamycin) pathway is often involved in tumorgenesis and in sensitivity and resistance to cancer therapy. Deregulated signaling through the PI3K/PTEN/Akt/mTOR pathway is often the result of genetic alterations in critical components of this pathway and/or mutations at upstream growth factor receptors or signaling components. PI3K initiates a cascade of events when activated by, for example, extracellular growth factors, mitogens, cytokines and/or receptors, PDK1 activates Akt, which in turn phosphorylates and inactivates the tumor suppressor complex comprising TSC1 and 2 (tuberous sclerosis complex 1/2), resulting in the activation of mTORC1 (target of rapamycin complex 1) by Rheb-GTP. Activation of PDK1 and Akt by PI3Ks is negatively regulated by PTEN.
PTEN is a critical tumor suppressor gene and is often mutated or silenced in human cancers. Its loss results in activation of Akt and increases downstream mTORC1 signaling. The involvement of mTOR complex 1 (mTORC1) in neoplastic transformation appears to depend on its regulatory role toward the eIF4F complex; overexpression of eIF4E can confer resistance to rapamycin. mTORC1 regulates the eIF4F complex assembly that is critical for the translation of mRNAs associated with cell growth, prevention of apoptosis and transformation. mTORC1 achieves this by phosphorylation and inactivation of 4E-BPs and the subsequent dissociation of 4E-BPs from eIF4E. This then enables eIF4E to interact with the scaffold protein eIF4G, permitting assembly of the eIF4F complex for the translation of structured mRNAs. mTORC1 also promotes activation of the translational activator, S6K, which phosphorylates the ribosomal protein S6 and other substrates, including eIF4B. mTORC1 signaling is inhibited by rapamycin and its analogues (rapalogs), although these compounds act allosterically, rather than directly inhibiting mTOR kinase activity.
Given the importance of the PI3K/Akt/mTOR pathway in regulating mRNA translation of genes that encode for pro-oncogenic proteins and activated mTORC1 signaling in a high proportion of cancers, these kinases have been actively pursued as oncology drug targets. A number of pharmacological inhibitors have been identified, some of which have reached advanced clinical stages. However, it has recently become clear that the mTOR pathway participates in a complicated feedback loop that can impair activation of Akt. It has been shown that prolonged treatment of cancer cells or patients with mTOR inhibitors causes elevated PI3K activity that leads to phosphorylation of Akt and eIF4E, and promotes cancer cell survival. eIF4E, acting downstream of Akt and mTOR, recapitulates Akt's action in tumorigenesis and drug resistance, and Akt signaling via eIF4E is an important mechanism of oncogenesis and drug resistance in vivo.
In addition to the PI3K/Akt/mTOR pathway, eIF4E is also the target of the Ras/Raf/MAP signaling cascade which is activated by growth factors and for the stress-activated p38 MAP kinase pathway. Erk1/2 and p38 then phosphorylate MAP kinase-interacting kinase 1 (Mnk1) and MAP kinase-interacting kinase 2 (Mnk2). The Erk pathway is also activated in many cancers, reflecting, for example, activating mutations in Ras (found in around 20% of tumors) or loss of function of the Ras GTPase-activator protein NF1. Mnk1 and Mnk2 are threonine/serine protein kinases and specifically phosphorylate serine 209 (Ser209) of eIF4E within the eIF4F complex, by virtue of the interaction between eIF4E and the Mnks, which serves to recruit Mnks to act on eIF4E. Mice with mutated eIF4E, in which Ser209 is replaced by alanine, show no eIF4E phosphorylation and significantly attenuated tumor growth. Significantly, while Mnk activity is necessary for eIF4E-mediated oncogenic transformation, it is dispensable for normal development. Pharmacologically inhibiting Mnks thus presents an attractive therapeutic strategy for cancer.
Despite increased understanding of Mnk structure and function, little progress has been made with regard to the discovery of pharmacological Mnk inhibitors and relatively few Mnk inhibitors have been reported: CGP052088 (Tschopp et al., Mol Cell Biol Res Commun. 3(4):205-211, 2000); CGP57380 (Rowlett et al., Am J Physiol Gastrointest Liver Physiol. 294(2):G452-459, 2008); and Cercosporamide (Konicek et al., Cancer Res. 71(5):1849-1857, 2011). These compounds, however, have mainly been used for the purpose of Mnk target validation. More recently, investigators have proposed further compounds for treating diseases influenced by the inhibition of kinase activity of Mnk1 and/or Mnk2, including, for example, the compounds disclosed in WO 2014/044691 and the various patent documents cited therein and the 4-(dihydropyridinon-3-yl)amino-5-methylthieno[2,3,-d]pyrimidines disclosed by Yu et al., European Journal of Med. Chem., 95: 116-126, 2015.
Accordingly, while advances have been made in this field there remains a significant need in the art for compounds that specifically inhibit Mnk kinase activity, particularly with regard to Mnk's role in regulation of cancer pathways, as well as for associated composition and methods. The present invention fulfills this need and provides further related advantages.